Implantable medical devices are available to provide therapies for restoring normal cardiac rhythms by delivering electrical shock therapy for cardioverting or defibrillating the heart in addition to cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, senses a patient's heart rhythm and classifies the rhythm according to a number of rate zones in order to detect episodes of tachycardia or fibrillation. Single chamber devices are available for treating either atrial arrhythmias or ventricular arrhythmias, and dual chamber devices are available for treating both atrial and ventricular arrhythmias. Rate zone classifications may include slow tachycardia, fast tachycardia, and fibrillation.
Upon detecting an abnormal rhythm, the ICD delivers an appropriate therapy. Cardiac pacing is delivered in response to the absence of sensed intinsic depolarizations, referred to as P-waves in the atrium and R-waves in the ventricle. In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and escalating to more aggressive shock therapies until the tachycardia is terminated. Termination of a tachycardia is commonly referred to as “cardioversion.” Ventricular fibrillation (VF) is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy. Termination of VF is normally referred to as “defibrillation.”
In modern implantable cardioverter defibrillators, the physician programs the particular anti-arrhythmia therapies into the device ahead of time, and a menu of therapies is typically provided. For example, on initial detection of an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be selected and delivered to the chamber in which the tachycardia is diagnosed or to both chambers. On redetection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled. If repeated attempts at anti-tachycardia pacing therapies fail, a higher energy cardioversion pulse may be selected. For an overview of tachycardia detection and treatment therapies reference is made to U.S. Pat. No. 5,545,186 issued to Olson et al.
Detection of tachycardia or fibrillation may also trigger the storage of the sensed intracardiac electrogram (EGM) for a period of several seconds such that the EGM signals leading up to and during a detected arrhythmia episode are available for downloading and displaying on an external programmer or other device for analysis by a physician. Such analysis aids the physician in monitoring the status of the patient and the patient's response to delivered therapies. Occasionally, cardioversion or defibrillation therapies are delivered when the patient does not feel symptomatic. In such cases, the ICD may inappropriately detect a tachycardia or fibrillation episode that does not exist and deliver an anti-arrhythmia therapy when it is not needed. Inappropriate arrhythmia detections may cause a patient to experience painful, repeated shocks within a short period of time. Anti-tachycardia pacing therapies delivered during normal sinus rhythm can potentially induce an arrhythmia in some patients. For these reasons, the delivery of a therapy in response to inappropriate arrhythmia detection is highly undesirable.
Inappropriate arrhythmia detection is generally caused by oversensing. Oversensing can be defined as the sensing of events other than the one P-wave and/or the one R-wave occurring during each normal sinus cardiac cycle. Oversensing of both cardiac and non-cardiac events can result in inappropriate arrhythmia detection by the ICD if the detected rate due to oversensing falls into an arrhythmia detection zone. Cardiac oversensing refers to oversensing of cardiac events such as far-field R-waves, T-waves, or R-waves that are sensed twice and are therefore “double-counted”. Examples of cardiac oversensing are illustrated in FIG. 1. A conventional ECG signal is illustrated showing a normal cardiac cycle indicated by a P-wave, R-wave, and T-wave. Beneath the ECG, is a typical ventricular intracardiac electrogram signal (VEGM) in which a ventricular signal spike coincides with the R-wave on the ECG. During normal sensing, shown beneath the VEGM, one atrial sensed event (AS) and one ventricular sensed event (VS) occur for each cardiac cycle, corresponding to the atrial P-wave and the ventricular R-wave, respectively.
Far-field R-wave oversensing is illustrated in FIG. 1 in which one atrial sensed event (AS) per cardiac cycle corresponds to the normal P-wave and a second atrial sensed event (AS) per cardiac cycle corresponds to the R-wave. Far-field R-waves are sometimes sensed in the atria because the amplitude of an R-wave, as sensed at the atrial sensing electrodes, can reach the atrial sensitivity threshold. Therefore an atrial sensitivity setting required for sensing P-waves may also result in sensing of far-field R-waves from the ventricles.
T-wave oversensing is illustrated in FIG. 1 in which two ventricular sensed events (VS) occur during each cardiac cycle, one coinciding with the R-wave and one coinciding with the T-wave. T-wave oversensing occurs when the ventricular sensitivity setting is too sensitive, resulting in sensing of both R-waves and T-waves. T-wave oversensing also occurs when the R-wave amplitude has reduced to a point that causes the auto-adjusting threshold, which is a function of the R-wave, to decrease below the T-wave threshold. R-wave oversensing, also referred to as “R-wave double-counting,” is also illustrated in FIG. 1 in which two ventricular sense events (VS) correspond to one R-wave. This “double-counting” of R-waves can occur, for example, when an R-wave complex is widened due to conditions such as bundle branch block or wide complex ventricular tachycardia. For each of these types of cardiac oversensing, generally one extra atrial or ventricular sensed event occurs per cardiac cycle, as seen in the illustrations of FIG. 1.
Non-cardiac oversensing refers to undesired sensing of other electrical signals by an ICD that are not cardiac in origin. Such non-cardiac signals are generally referred to as “noise.” Noise may occur in the form of myopotentials from surrounding muscle tissue or as the result of electromagnetic interference (EMI) external to the patient. Noise may also occur when the insulation of a lead fails, a lead conductor becomes fractured, or when a lead is poorly connected to the ICD.
Examples of non-cardiac oversensing are illustrated in FIGS. 2A through 2C. In FIG. 2A, a ventricular EGM (VEGM) signal is shown with a corresponding illustration of EMI oversensing. EMI appears as relatively continuous high frequency noise on the VEGM and can be repeatedly sensed as a ventricular event (VS) by the ICD. In FIG. 2B, a ventricular EGM (VEGM) is shown with a corresponding illustration of myopotential oversensing. Myopotentials may appear as lower frequency noise on the VEGM than EMI, resulting in somewhat less frequent but repeated ventricular sensed events (VS). In FIG. 2C, a ventricular EGM (VEGM) is shown corresponding to noise associated with a lead fracture or a poor lead connection. This type of noise can result in saturation of the sense amplifiers and intermittent bursts of noise. Oversensing due to a lead fracture or poor lead connection, therefore, produces intermittent clusters of ventricular sensed events (VS), as shown in FIG. 2C. As seen in FIGS. 2A through 2C, non-cardiac oversensing is generally associated with multiple oversensed events per cardiac cycle that may be intermittent or continuous, of high or low amplitude, and of relatively low or high frequency.
Since these problems of oversensing can be rare and are therefore not routinely encountered in all patients, the task of recognizing and trouble-shooting oversensing can be a challenging one to the physician. Oversensing may not be recognized until inappropriate arrhythmia detections are made and unneeded therapies are delivered. While stored EGM data can be useful in identifying and trouble-shooting inappropriate arrhythmia detections due to oversensing, valid arrhythmia detections may occur the majority of the time with only an occasional inappropriate detection occurring, making the identification of EGM episodes associated with inappropriate detections a time-consuming task. Once an inappropriate detection is identified, the numerous types of oversensing that may have caused the detection make diagnosing the problem complex. With a growing number of ICD patients in broad geographical distributions, clinicians need to be able to quickly and confidently diagnose and correct such problems. What is needed, therefore, is an automated method for recognizing oversensing and specifically identifying the type of oversensing present so that a physician may make prompt corrective actions with confidence.